linux-kernel - Re: Alternatives to /sys/kernel/debug/wakeup

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Date:   Wed, 19 Jun 2019 14:55:16 -0400
From:   Joel Fernandes <joelaf@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Tri Vo <trong@...roid.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Sandeep Patil <sspatil@...roid.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexei Starovoitov <ast@...com>
Subject: Re: Alternatives to /sys/kernel/debug/wakeup_sources

On Wed, Jun 19, 2019 at 2:35 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Wed, Jun 19, 2019 at 02:01:36PM -0400, Joel Fernandes wrote:
> > On Wed, Jun 19, 2019 at 1:07 PM Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org> wrote:
> > >
> > > On Wed, Jun 19, 2019 at 12:53:12PM -0400, Joel Fernandes wrote:
> > > > > It is conceivable to have a "wakeup_sources" directory under
> > > > > /sys/power/ and sysfs nodes for all wakeup sources in there.
> > > >
> > > > One of the "issues" with this is, now if you have say 100 wake up
> > > > sources, with 10 entries each, then we're talking about a 1000 sysfs
> > > > files. Each one has to be opened, and read individually. This adds
> > > > overhead and it is more convenient to read from a single file. The
> > > > problem is this single file is not ABI. So the question I guess is,
> > > > how do we solve this in both an ABI friendly way while keeping the
> > > > overhead low.
> > >
> > > How much overhead?  Have you measured it, reading from virtual files is
> > > fast :)
> >
> > I measured, and it is definitely not free. If you create and read a
> > 1000 files and just return a string back, it can take up to 11-13
> > milliseconds (did not lock CPU frequencies, was just looking for
> > average ball park). This is assuming that the counter reading is just
> > doing that, and nothing else is being done to return the sysfs data
> > which is probably not always true in practice.
> >
> > Our display pipeline deadline is around 16ms at 60Hz. Conceivably, any
> > CPU scheduling competion reading sysfs can hurt the deadline. There's
> > also the question of power - we definitely have spent time in the past
> > optimizing other virtual files such as /proc/pid/smaps for this reason
> > where it spent lots of CPU time.
>
> smaps was "odd", but that was done after measurements were actually made
> to prove it was needed.  That hasn't happened yet :)
>
> And is there a reason you have to do this every 16ms?

Not every, I was just saying whenever it happens and a frame delivery
deadline is missed, then a frame drop can occur which can result in a
poor user experience.